We plan to investigate factors involved in the fidelity of DNA synthesis. Mutants showing unusually high spontaneous mutation rates (mutators) and those showing unusually low spontaneous mutation rates (antimutators) will be investigated to determine the biochemical basis for these aberrations. The mechanisms involved in monitoring nucleotides eventually incorporated into the DNA chain will be investigated using 2-aminopurine deoxynucleoside triphosphate. An in vitro test for mutators and antimutators will be developed.